Musnad Ahmad 32:820

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard 'Attab, the freed slave of Ibn Hurmuz, say: I heard Anas ibn Malik say: "I pledged allegiance to the Messenger of Allah, may Allah bless him and grant him peace, with this hand of mine—meaning the right one—to hear and to obey, as much as I am able."